All animals show some form of learning, but the range, subtlety and pervasiveness of human learning is the main thing that sets human beings apart from other species. There are two ways to approach the learning problem: bottom up, and top down. The top-down method is to begin with careful behavioral analysis of apparently complex tasks that can nevertheless be reduced to simple performance rules. Theoretical analysis can often suggest formal real-time models that behave in the ways described by a given performance rule. A dynamic model of this sort that survives rigorous behavioral testing is likely to reflect enduring and measurable properties of the underlying neural machinery. This is an application for fellowship (RSA) support for the Principal Investigator. This PI's research comprises two linked projects that use the top-down approach to understand the mechanisms of learning. One project, Models for Learning, Memory and Inference is primarily theoretical and collaborative; the other project, Reinforcement Mechanisms is primarily experimental. The theoretical project has two parts: the study of general properties of operant learning, such as contingency and contiguity, serial order and simple associativity; and complex tasks, such as nonverbal inference. We have developed real-time models for the assignment-of-credit problem in operant learning for a simple kind of nonverbal inference and for different kinds of serial order in behavior. The experimental work uses animal subjects to study the role of dynamic memory processes in time discrimination and choice. A key feature of the research strategy is continual interplay between experiment and real-time models of the processes under study. Our immediate objective is to arrive at dynamic models for specific experimental situations. We then attempt to extend and combine these models to embrace as wide as possible a range of related tasks. Our ultimate objective is to use real-time models as guides to understanding the role of the nervous system in learning. The most intractable and disturbing human behavior disorders involve the systems for memory and learning. The proposed work is intended to increase our basic understanding of how these systems work, hence aid in the search for means to eliminate or alleviate memory and learning disorders.